BATSE Soft Gamma‐Ray Observations of GRO J0422+32

We report results of a comprehensive study of the soft gamma-ray (30 keV to1.7 MeV) emission of GROJ0422+32 during its first known outburst in 1992. Theseresults were derived from the BATSE earth-occultation database with the JPLdata analysis package, EBOP (Enhanced BATSE Occultation Package). Resultspresented here focus primarily on the long-term temporal and spectralvariability of the source emission associated with the outburst. The lightcurves with 1-day resolution in six broad energy-bands show the high-energyflux (>200 keV) led the low-energy flux (<200 keV) by ~5 days in reaching theprimary peak, but lagged the latter by ~7 days in starting the declining phase.We confirm the "secondary maximum" of the low-energy (<200 keV) flux at TJD8970-8981, ~120 days after the first maximum. Our data show that the "secondarymaximum" was also prominent in the 200-300 keV band, but became less pronouncedat higher energies. During this 200-day period, the spectrum evolved from apower-law with photon index of 1.75 on TJD 8839, to a shape that can bedescribed by a Comptonized model or an exponential power law below 300 keV,with a variable power-law tail above 300 keV. The spectrum remained roughly inthis two-component shape until ~9 November (TJD 8935) and then returned to theinitial power-law shape with an index of ~2 until the end of the period. Thecorrelation of the two spectral shapes with the high and low luminosities ofthe soft gamma-ray emission is strongly reminiscent of that seen in Cygnus X-1.We interpret these results in terms of the Advection Dominated Accretion Flow(ADAF) model with possibly a "jet-like" region that persistently produced thenon-thermal power-law gamma rays observed throughout the event.Comment: 40 pages total, including 10 figures and 2 table